Sacado de este enlace: 

https://www.t38fax.com/support/knowledgebase/cisco-spa112-2-port-phone-adapter/

We recommend updating to the latest firmware for the Cisco SPA112 before connecting to our service. It might be available here.

Issues can occur with fax transmissions over IP networks, even with the T.38 standard, which is supported by the ATA. You can adjust several settings on your ATA to optimize your fax completion rates.

STEP 1: Ensure that you have enough bandwidth for the uplink and the downlink.

• For G.711 fallback, it is recommend to have approximately 100 kbps.
• For T.38, allocate at least 50 kbps.

STEP 2: Click Voice in the menu bar, and then click Line 1 or Line 2 in the navigation tree.

STEP 3: In the Network Settings section, enter the following settings:

• Network Jitter Level: very high
• Jitter Buffer Adjustment: no

STEP 4: In the Supplementary Service Subscription section, enter the following settings:

• Call Waiting Serv: no
• Three Way Call Serv: no

STEP 5: In the Audio Configuration section, enter the following settings to support T.38 fax:

• Preferred Codec: G.711u (USA) or G.711a (rest of the world)
• Use pref. codec only: yes
• Silence Supp Enable: no
• Echo Canc Enable: no
• FAX Passthru method REINVITE
• FAX Enable T38: Yes
• FAX T38 ECM Enable: Yes
• Fax Tone Detect Mode: Callee only

To remind firewalls to keep the connection between the adapter and our service, we recommend:

redirect-keepalive yes