Update AR and MX FAQ (#1806)

FAQ.md

@@ -106,8 +106,8 @@ call or send an SMS to.
 M2M numbers would violate this assumption and we'd have to evaluate the
 consequences for existing APIs and clients if M2M numbers would be considered
 valid by the library. Clients of libphonenumber expect `mobile` and `fixed-line`
-numbers to have certain affordances, such as: Reachable for voice calls 
-(and for mobile also SMS) as well as assuming standard cost. This expectation 
+numbers to have certain affordances, such as: Reachable for voice calls
+(and for mobile also SMS) as well as assuming standard cost. This expectation
 is broken by the lack of M2M standardization today.
 
 Many people use this library for formatting the numbers of their contacts, for
@@ -211,28 +211,51 @@ to achieve this, please do so. Thanks!
 ### Why is this number from Argentina (AR) or Mexico (MX) not identified as the right number type?
 
 Certain countries' mobile and/or fixed line ranges may overlap, which may make
-accurate identification impossible without additional and explicit context
-such as a mobile prefix. We rely on this prefix being present to correctly identify
-the phone number type (rather than returning ['FIXED_LINE_OR_MOBILE'](#fixed_line_or_mobile)
-in ambiguous cases) until our metadata can be fine-grained enough to detect when a user has omitted it.
+accurate identification impossible without additional and explicit context such
+as a mobile prefix. We rely on this prefix being present to correctly identify
+the phone number type (rather than returning
+[`FIXED_LINE_OR_MOBILE`](#fixed_line_or_mobile) in ambiguous cases) until our
+metadata can be fine-grained enough to detect when a user has omitted it.
 
-For example, when calling a mobile line from a fixed line in Argentina,
-you need to dial 15 before the subscriber number, or 9 if you're calling
-from another country. Without these additional digits, your call may not
-connect at all!
+For example, when calling a mobile line from a fixed line in Argentina, you need
+to dial 15 before the subscriber number, or 9 if you're calling from another
+country. Without these additional digits, your call may not connect at all!
 
 Similarly, Mexico has different mobile prefixes needed when calling from a fixed
 line such as 044 when calling locally, 045 when calling from another state, and
 1 when dialing from another country.
 
-Moreover, these countries have different possible lengths for area codes and subscriber
-numbers depending on the city, which further complicate matters (e.g. Buenos Aires is 11
-followed by eight digits, but Río Gallegos is 2966 followed by six digits).
-
-Despite all the aforementioned complexity, users may not provide their phone number
-with all the additional context unless explicitly asked. For instance,
-since SMS messages can be sent in Argentina from a mobile phone without a prefix,
-the user may not supply the mobile prefix.
+Moreover, these countries have different possible lengths for area codes and
+subscriber numbers depending on the city, which further complicate matters (e.g.
+Buenos Aires is 11 followed by eight digits, but Río Gallegos is 2966 followed
+by six digits).
+
+Despite all the aforementioned complexity, users may not provide their phone
+number with all the additional context unless explicitly asked. For instance,
+since SMS messages can be sent in Argentina from a mobile phone without a
+prefix, the user may not supply the mobile prefix.
+
+We are aware of these issues but fixing them is not trivial. In the meantime, we
+recommend the following workarounds to support affected users.
+
+*   If you know an Argentina or Mexico number is mobile (e.g. if you're doing
+    signups with device numbers or will send them an SMS verification code),
+    follow these steps:
+    *   For raw input strings:
+        *   Parse a raw input string into a `PhoneNumber` and follow the next
+            instructions for `PhoneNumber` objects.
+    *   For `PhoneNumber` objects:
+        *   Check that the library validates a `PhoneNumber` as mobile, by
+            calling `getNumberType`;
+        *   If not, format it in national format and prepend a `9` for Argentina
+            or a `1` for Mexico;
+        *   Parse the modified string and if the library validates it as mobile,
+            accept the resulting `PhoneNumber` as canonical.
+*   Consider prompting for type (mobile or not) in the phone number input UI.
+
+IMPORTANT: Do not add a leading 1 or 9 for displaying or formatting the numbers.
+Depending on the use case, other tokens may be needed. The library will do the
+right thing if the phone number object is as intended.
 
 ### Why are Bouvet Island (BV), Pitcairn Island (PN), Antarctica (AQ) etc. not supported?