What this code means, when talking to another freespeech account the call is getting interrupted and the phone shows "Voip status code 701"

from SIP specifications

3.4.1 Warning code 701 Location Format Not Supported

A Warning header with the code 701 "Location Format not supported"
means the location format supplied in the request, by-value or
by-reference, was not supported. This cause means the recipient
understood that location was included in the message, but the format
is not supported. Perhaps the format was a freeform text format or
data-URL and the recipient only understood location in RFC 4119
PIDF-LO format (civic or coordinate). If a more specific Warning
code is available, it MUST be used. For example, if the format is
understood, but not desired, a 702 or 703 Warning header should be
returned in a 424 response, depending on which location format is
desired. The same applies to a location recipient that only
understands one format and did not receive that format. For example,
if a message containing coordinate formatted location arrives but
the recipient only can process civic formatted location, a 703
Warning header should be returned in a 424 response.

Recommended warn-text: Location format not supported

An example usage in a SIP 424 response:

Warning: 701 alice.example.com "Location Format not supported"

this would be between clients and not from our service