yeah definitely call the recipient with a regular phone and listen for the handshake
Is this a number you’ve sent to before? Try one that worked before or check with the recipient to see if they hear your tones. Have them send you a fax if possible.
Could be sending to an efax configured improperly. All tones for send and receive should be in-band.
VoIP networks are configurable for tone treatment as well. Could have something to do with recipient’s internal phone system.