Can you use different Voltage and Amps to a charge phone?
If you try to charge your 5V phone with 18V, you will FRY IT. NOT RECOMMENDED!

If you regulate the voltage down and try to charge your 5V phone with 5V @ 250~300 mA it will take 4~5x longer to charge vs. the specified 1A.

Yes, there are ways to change 18V into 5V with some boost in current. However such DC-DC converters lose some significant power because they are not 100% efficient.

An 18V @ 300mA solar panel is a POOR MATCH to a 5V @ 1A load. Probably a waste of money.  Find a solar panel that is closer to what your load needs.