better erorr messages for omitted commas in INVITE commands

STUB

current state:

INVITE +1-555-555-5555 +1-555-555-555

yields

"+1-555-555-5555+1-555-555-5555" is not a valid phone number. Phone numbers must include country codes prefixed by a '+'.

we should at least mention that we expected commas and maybe consider something more full-blown like suggested in #352 (which perhaps could be combined with this card?)

Edited Sep 24, 2020 by aguestuser

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