- Is SIP ALG disabled on router

- Can the issue be replicated from a soft phone or another IP device

- Can the issue be replicated from a different network

- Is the IP phone/softphone on the latest firmware

- Is the speaker cord in the right port in the base of the device

- Does the same issue occur on loud speaker

- Does the issue occur on one specific number or multiple

- Does the issue occur on every call

- Is it incoming/outgoing calls or both