There are ways to bond via telco's cooperation. If the ISP let's you for MLPPP, then your DSL lines can be bonded.
If however, your ISP doesn't support MLPPP (which is almost always the case other than some exception), you can look into "broadband bonding" as an alternative. Broadband Bonding is true bonding (it is not load-balancing sessions, it is indeed bonding at a packet level) and doesn't require any coordination from the ISP. Additionally, with this method, you can bond different ISP connections.
For bonding cellular aircards, check out what this film production company did: