Question: Can my external antenna cause my neighbours to lose their mobile voice signal?
Answer: Simple answer is that there is no chance of interfering with your neighbours voice (or data) communications:
The frequency bands used for voice calls and data transmission differs from one service provider to another. The service providers usually pay a significant amount of money to operate in their own frequency bands and this is to ensure that there is no interference from other service providers.
Even if your neighbour was using the same service provider for their voice calls that you use for your data, it would still not affect their signal. This is because service providers use different frequency bands for voice and data. In time they may use the same bands, but once again the access is not controlled from the user device but from the base stations. This is to make sure that nobody interferes, unless they use damaged, broken or uncertified equipment.
Any equipment transmitting RF (for instance your cellphone or modem) should be certified by the local certification body in your country. This is to ensure that all equipment adhere to the ETSI- and CISPR-standards and do not interfere with other radio systems or other electrical systems and isn’t harmful to your health and environment.
Outdoor antennas are “passive” devices which cannot “alter/degrade” the modem signal parameters. The use of an outdoor antenna will improve the performance of the modem by removing radiation indoors to outdoors, resulting in an improved signal quality. This will in turn further reduce any effect of interference from the neighbour.