Are you using the FSX port to plug your modem into from Ubuntu?

The HT-701 would log into your SIP account (whether it's an IP-PBX or an
ITSP) to accept phone calls and route them to the FXS port.  

I use an HT802 using a SIP account through our company IP-PBX (Grandstream
UCM6510) for our Konica 454 copy machine to receive faxes, works well so far.

I've never used it to carry modem signals but I imagine it would work without
having to change too much by way of CODECs, might have to experiment with
T.38 on or off, even though that's for Faxing, it is similar to computer
modems as far as signaling goes.

Good luck!

--- Mystic BBS v1.12 A39 2018/04/21 (Linux/64)